Each of Subversion's core libraries can be said to
exist in one of three main layers—the Repository layer,
the Repository Access (RA) layer, or the Client layer (see
Figure 1, “Subversion's architecture” in the Preface).
We will examine these layers shortly, but first, let's briefly
summarize Subversion's various libraries. For the sake of
consistency, we will refer to the libraries by their
extensionless Unix library names
Primary interface for client programs
Tree and byte-stream differencing routines
Contextual differencing and merging routines
Filesystem commons and module loader
The Berkeley DB filesystem backend
The native filesystem (FSFS) backend
Repository Access commons and module loader
The local Repository Access module
Another (experimental) WebDAV Repository Access module
The custom protocol Repository Access module
Miscellaneous helpful subroutines
The working copy management library
Apache authorization module for Subversion repositories access via WebDAV
Apache module for mapping WebDAV operations to Subversion ones
The fact that the word “miscellaneous” appears only once in the previous list is a good sign. The Subversion development team is serious about making sure that functionality lives in the right layer and libraries. Perhaps the greatest advantage of the modular design is its lack of complexity from a developer's point of view. As a developer, you can quickly formulate that kind of “big picture” that allows you to pinpoint the location of certain pieces of functionality with relative ease.
Another benefit of modularity is the ability to replace a
given module with a whole new library that implements the same
API without affecting the rest of the code base. In some sense,
this happens within Subversion already. The
libsvn_ra_svn libraries each implement the
same interface, all working as plug-ins to
libsvn_ra. And all three communicate with
the Repository layer—
connects to the repository directly; the others do so over a
libsvn_fs_fs libraries are another pair of
libraries that implement the same functionality in different
ways—both are plug-ins to the common
The client itself also highlights the benefits of modularity
in the Subversion design. Subversion's
libsvn_client library is a one-stop shop
for most of the functionality necessary for designing a working
Subversion client (see the section called “Client Layer”). So while the
Subversion distribution provides only the svn
command-line client program, several third-party
programs provide various forms of graphical client UIs.
These GUIs use the same APIs that the stock command-line client
does. This type of modularity has played a large role in the
proliferation of available Subversion clients and IDE
integrations and, by extension, to the tremendous adoption rate
of Subversion itself.
When referring to Subversion's Repository layer, we're
generally talking about two basic concepts—the versioned
filesystem implementation (accessed via
libsvn_fs, and supported by its
libsvn_fs_fs plug-ins), and the repository
logic that wraps it (as implemented in
libsvn_repos). These libraries provide
the storage and reporting mechanisms for the various revisions
of your version-controlled data. This layer is connected to
the Client layer via the Repository Access layer, and is, from
the perspective of the Subversion user, the stuff at the
“other end of the line.”
The Subversion filesystem is not a kernel-level filesystem that one would install in an operating system (such as the Linux ext2 or NTFS), but instead is a virtual filesystem. Rather than storing “files” and “directories” as real files and directories (the kind you can navigate through using your favorite shell program), it uses one of two available abstract storage backends—either a Berkeley DB database environment or a flat-file representation. (To learn more about the two repository backends, see Speaking of Filesystems….) There has even been considerable interest by the development community in giving future releases of Subversion the ability to use other backend database systems, perhaps through a mechanism such as Open Database Connectivity (ODBC). In fact, Google did something similar to this before launching the Google Code Project Hosting service: they announced in mid-2006 that members of its open source team had written a new proprietary Subversion filesystem plug-in that used Google's ultra-scalable Bigtable database for its storage.
The filesystem API exported by
libsvn_fs contains the kinds of
functionality you would expect from any other filesystem
API—you can create and remove files and directories,
copy and move them around, modify file contents, and so on.
It also has features that are not quite as common, such as the
ability to add, modify, and remove metadata
(“properties”) on each file or directory.
Furthermore, the Subversion filesystem is a versioning
filesystem, which means that as you make changes to your
directory tree, Subversion remembers what your tree looked
like before those changes. And before the previous changes.
And the previous ones. And so on, all the way back through
versioning time to (and just beyond) the moment you first
started adding things to the filesystem.
All the modifications you make to your tree are done within the context of a Subversion commit transaction. The following is a simplified general routine for modifying your filesystem:
Begin a Subversion commit transaction.
Make your changes (adds, deletes, property modifications, etc.).
Commit your transaction.
Once you have committed your transaction, your filesystem modifications are permanently stored as historical artifacts. Each of these cycles generates a single new revision of your tree, and each revision is forever accessible as an immutable snapshot of “the way things were.”
Most of the functionality the filesystem
interface provides deals with actions that occur on individual
filesystem paths. That is, from outside the filesystem, the
primary mechanism for describing and accessing the individual
revisions of files and directories comes through the use of
path strings such as
/foo/bar, just as though
you were addressing files and directories through your
favorite shell program. You add new files and directories by
passing their paths-to-be to the right API functions. You
query for information about them by the same mechanism.
Unlike most filesystems, though, a path alone is not enough information to identify a file or directory in Subversion. Think of a directory tree as a two-dimensional system, where a node's siblings represent a sort of left-and-right motion, and navigating into the node's subdirectories represents a downward motion. Figure 8.1, “Files and directories in two dimensions” shows a typical representation of a tree as exactly that.
The difference here is that the Subversion filesystem has
a nifty third dimension that most filesystems do not
have—Time! In the filesystem
interface, nearly every function that has a
path argument also expects a
root argument. This
svn_fs_root_t argument describes
either a revision or a Subversion transaction (which is simply
a revision in the making) and provides that third dimension
of context needed to understand the difference between
/foo/bar in revision 32, and the same
path as it exists in revision 98. Figure 8.2, “Versioning time—the third dimension!” shows revision
history as an added dimension to the Subversion filesystem
As we mentioned earlier, the
libsvn_fs API looks and feels like any
other filesystem, except that it has this wonderful versioning
capability. It was designed to be usable by any program
interested in a versioning filesystem. Not coincidentally,
Subversion itself is interested in that functionality. But
while the filesystem API should be sufficient for basic file
and directory versioning support, Subversion wants
more—and that is where
The Subversion repository library
libsvn_repos) sits (logically speaking)
libsvn_fs API, providing
additional functionality beyond that of the underlying
versioned filesystem logic. It does not completely wrap each
and every filesystem function—only certain major steps
in the general cycle of filesystem activity are wrapped by the
repository interface. Some of these include the creation and
commit of Subversion transactions and the modification of
revision properties. These particular events are wrapped by
the repository layer because they have hooks associated with
them. A repository hook system is not strictly related to
implementing a versioning filesystem, so it lives in the
repository wrapper library.
The hooks mechanism is but one of the reasons for the
abstraction of a separate repository library from the rest of
the filesystem code. The
API provides several other important utilities to Subversion.
These include the abilities to:
Create, open, destroy, and perform recovery steps on a Subversion repository and the filesystem included in that repository.
Describe the differences between two filesystem trees.
Query for the commit log messages associated with all (or some) of the revisions in which a set of files was modified in the filesystem.
Generate a human-readable “dump” of the filesystem—a complete representation of the revisions in the filesystem.
Parse that dump format, loading the dumped revisions into a different Subversion repository.
As Subversion continues to evolve, the repository library will grow with the filesystem library to offer increased functionality and configurable option support.
If the Subversion Repository layer is at “the other
end of the line,” the Repository Access (RA) layer is
the line itself. Charged with marshaling data between the
client libraries and the repository, this layer includes the
libsvn_ra module loader library, the RA
modules themselves (which currently includes
libsvn_ra_svn), and any additional
libraries needed by one or more of those RA modules (such as
mod_dav_svn Apache module or
Since Subversion uses URLs to identify its repository
resources, the protocol portion of the URL scheme (usually
svn+ssh://) is used to determine which RA
module will handle the communications. Each module registers
a list of the protocols it knows how to “speak”
so that the RA loader can, at runtime, determine which module
to use for the task at hand. You can determine which RA
modules are available to the Subversion command-line client,
and what protocols they claim to support, by running
$ svn --version svn, version 1.8.0-dev (under development) compiled Jan 8 2013, 11:45:25 on i686-pc-linux-gnu Copyright (C) 2013 The Apache Software Foundation. This software consists of contributions made by many people; see the NOTICE file for more information. Subversion is open source software, see http://subversion.apache.org/ The following repository access (RA) modules are available: * ra_svn : Module for accessing a repository using the svn network protocol. - with Cyrus SASL authentication - handles 'svn' scheme * ra_local : Module for accessing a repository on local disk. - handles 'file' scheme * ra_serf : Module for accessing a repository via WebDAV protocol using serf. - handles 'http' scheme - handles 'https' scheme $
The public API exported by the RA layer contains
functionality necessary for sending and receiving versioned
data to and from the repository. And each of the available RA
plug-ins is able to perform that task using a specific
libsvn_ra_serf speaks HTTP/WebDAV
(optionally using SSL encryption) with an Apache HTTP Server
that is running the
Subversion server module;
speaks a custom network protocol with the
svnserve program; and so on.
For those who wish to access a Subversion repository using still another protocol, that is precisely why the Repository Access layer is modularized! Developers can simply write a new library that implements the RA interface on one side and communicates with the repository on the other. Your new library can use existing network protocols or you can invent your own. You could use interprocess communication (IPC) calls, or—let's get crazy, shall we?—you could even implement an email-based protocol. Subversion supplies the APIs; you supply the creativity.
On the client side, the Subversion working copy is where all the action takes place. The bulk of functionality implemented by the client-side libraries exists for the sole purpose of managing working copies—directories full of files and other subdirectories that serve as a sort of local, editable “reflection” of one or more repository locations—and propagating changes to and from the Repository Access layer.
Subversion's working copy library,
libsvn_wc, is directly responsible for
managing the data in the working copies. To accomplish this,
the library stores administrative information about the
working copy within a special subdirectory. This
.svn, is present in
each working copy and contains various other files
and directories that record state and provide a private
workspace for administrative action. For those familiar with
.svn subdirectory is similar in
purpose to the
directories found in CVS working copies.
The Subversion client library,
libsvn_client, has the broadest
responsibility; its job is to mingle the functionality of the
working copy library with that of the Repository Access layer,
and then to provide the highest-level API to any application
that wishes to perform general revision control actions. For
example, the function
svn_client_checkout() takes a URL as an
argument. It passes this URL to the RA layer and opens an
authenticated session with a particular repository. It then
asks the repository for a certain tree, and sends this tree
into the working copy library, which then writes a full
working copy to disk (
The client library is designed to be used by any
application. While the Subversion source code includes a
standard command-line client, it should be very easy to write
any number of GUI clients on top of the client library. New
GUIs (or any new client, really) for Subversion need not be
clunky wrappers around the included command-line
client—they have full access via the
libsvn_client API to the same functionality,
data, and callback mechanisms that the command-line client
uses. In fact, the Subversion source code tree contains a
small C program (which you can find at
exemplifies how to wield the Subversion API to create a simple
 We understand that this may come as a shock to sci-fi fans who have long been under the impression that Time was actually the fourth dimension, and we apologize for any emotional trauma induced by our assertion of a different theory.